Garnet Gneiss Research Articles

High-pressure metamorphism of Precambrian continental crust in the southwestern part of the Central Asian Orogenic Belt (South Kazakhstan and North Tien Shan) and tectonic implications for the evolution of the Palaeo-Asian Ocean

AbstractIn this study, we investigated the high-pressure (HP) metamorphism of the Precambrian continental crust exposed in the Zheltau terrane in South Kazakhstan (Koyandy complex) and the Chu-Kendyktas terrane in the North Tien Shan of Kyrgyzstan (Aktyuz, Kemin and Kokdzhon complexes) within the SW part of the Central Asian Orogenic Belt. HP quartz–feldspar lithologies of the Koyandy complex consist of migmatized kyanite-bearing garnet–mica paragneisses, garnet–kyanite paragneisses and their derivatives associated with eclogites. Paragneisses demonstrate prograde evolution involving mica dehydration melting and producing magnesium-rich garnet, kyanite and K-feldspar at the near-peak to retrograde stages at pressures of 15–18.5 kbar and temperatures of 800–870°C. The widespread growth of micas in these rocks reflects lower stages of retrogression at P = 10–12 kbar and T = 720–770°C. The age distributions of the cores of detrital zircon grains from the paragneisses indicate a predominance of Neoproterozoic and minor occurrence of Mesoproterozoic and Palaeoproterozoic sources of their protoliths. The ages of ∼487–485 Ma obtained from the zircon rims of the paragneisses reflect the timing of their HP metamorphic re-equilibration. These age clusters are consistent with the age estimates obtained from the rims of zircons in the eclogite-bearing garnet gneisses of the adjacent Aktyuz complex in the North Tien Shan. The P–T paths and zircon ages obtained from the high-grade quartz–feldspar gneisses of the Zheltau and Chu-Kendyktas terranes are thus interpreted to indicate involvement of the crustal material derived from the Precambrian basement (magmatic zircons aged ca. 844 Ma) and its Ediacaran–Cambrian sedimentary cover (detrital zircons with maxima at 1 Ga and 800–600 Ma) in the latest Cambrian subduction processes induced by the closure of the oceanic basins assigned to the Palaeo-Asian Ocean.

Whole Rock Geochemistry and Geodynamic Evolution of Paleoproterozoic Gneisses, Ako’ozam-Njabilobe Area, Southwestern Cameroon

The Ako’ozam–Njabilobe area, which is part of the Nyong group, is characterized mainly by highly deformed gneisses that host amphibolites, and metadolerites as enclaves. The gneisses within the Ako’ozam-Njabilobe area were investigated to constrain their geotectonic environment as well as understand their geodynamic evolution within the Nyong group. Three litho-types of gneisses outcrop within the Ako’ozam-Njabilobe area, consisting of horblende-biotite, quartzofeldspathic, and garnet gneisses. They are strongly peraluminous, with low Yb<SUB>N</SUB>, Rb/Sr, K₂O/Na₂O, and enriched in large ion lithophile elements (LILE). The gneisses within the Ako’ozam-Njabilobe area have a trachy-andesite parentage. The negative anomalies in high field strength elements (HFSE) and enrichments in LILE and light rare earth elements (LREE) suggest a subduction setting for this magmatism. The geochemical signatures including, low K₂O/Na₂O ratio, silica content (67.18 to 73.65 wt%), Yb and Y contents less than 1 and 10 ppm respectively, and low heavy REE values of the studied gneisses are akin to chemical signatures of archean crustal rocks. The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination. The similarity in whole rock data of the three lithotypes of gneisses within the Ako’ozam-Njabilobe area suggests a possible mechanical mixing during emplacement. The chemistry of the gneisses indicates a possible Archean inheritance during the emplacement of their protoliths, as such could provide insights into the petrogenetic processes controlling crustal growth and aid in understanding the Archean-Proterozoic transition within the Nyong group.

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

The study is mainly focused on lithostratigraphy, petrography and geological structures to decipher the metamorphic facies and metamorphic history of the Khudi-Tal area along the Marsyangdi Valley, west central Nepal. Geological mapping was carried out covering 142 sq.km, on the scale of 1:50000. The Main Central Thrust (MCT) separates lithological succession of the Lesser Himalaya and Higher Himalaya around Bahundanda, Probhi and Naiche of the Khudi-Tal area. Lithological succession is dominated by metapelitic to psammatic schist, siliceous to dolomitic marble, quartzites, graphitic to garnet schist, kyanite garnet gneiss and banded gneiss from south to north. The kyanite isograd follow the MCT separating Kyanite zone with mineral assemblages Ky+Grt+Qtz+Ms+Bt+Pl of the Higher Himalaya and Garnet zone as Grt+Bt+Ms+Chl+K-Fel+Pl+Qtz in pelitic rocks, Bt+Ms+Chl+Plag+Qtz in Psammatic rocks and Cal/ Dol+Qtz+Pl+Bt+Ms in calcareous rock of the Lesser Himalaya. Based on the mineral assemblages, the Higher Himalaya belongs to Amphibolite Facies whereas the Lesser Himalaya belongs to Epidote-Amphibolite Facies. The lithological succession of the Higher and the Lesser Himalaya has undergone poly-phase metamorphism. In the Lesser Himalayan zone rocks, the regional burial metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type garnet grade inverted metamorphism M2 and later post-MCT retrogressive metamorphism M3. In the Higher Himalaya rocks, the early phase kyanite bearing medium temperature/ high pressure regional prograde metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type inverted metamorphism M3 (syn-MCT/ neo-Himalayan) and over printed by later phase retrogressive metamorphism M3 (post-MCT).

Pressure–temperature–time constraints on metamorphism in the southeastern Taltson Domain, Saskatchewan, Canada

The southeastern Taltson Domain is located on the southern margin of the Rae Craton in northern Saskatchewan, Canada, and encompasses the region south of the Athabasca Basin and west of the Snowbird Tectonic Zone. There has been relatively little metamorphic geochronology and no quantitative pressure–temperature (P–T) constraints from the southeastern Taltson Domain, which hinders attempts to link its metamorphic history to surrounding regions. New in situ LA-ICP-MS U–Pb monazite and zircon ages from rare metasedimentary rocks from the southeastern Taltson Domain reveal a polymetamorphic history. A cordierite–sillimanite–spinel-bearing migmatite from Fournier Lake records high thermal gradient metamorphism at 1988 ± 21 Ma, reaching P–T conditions of 4–6.4 kbar and > 900 °C. The cordierite–sillimanite–spinel assemblage overprints an earlier biotite–sillimanite-bearing assemblage that formed at 2075 ± 31 Ma. An orthopyroxene–garnet gneiss from within the Snowbird Tectonic Zone has a dominant monazite population at c. 1.90 Ga, but contains rare monazite ages between c. 2.19–2.01 Ga that support a phase of metamorphism prior to 2.0 Ga. The main metamorphic event to have affected the southeastern Taltson Doman occurred at c. 1.94–1.90 Ga. Phase equilibria modelling suggests peak P–T conditions were 8.3–11.2 kbar and 800–930 °C. Monazite with high HREE contents yields weighted mean ages of 1926–1918 Ma, interpreted to reflect prograde metamorphism prior to significant garnet growth. Zircon from an extremely garnet and zircon rich sample (now silicified and chloritized as a result of younger alteration) formed at c. 1918 Ma. REE compositions of the zircon indicate that it formed in equilibrium with garnet at temperatures of ∼ 830 °C, suggesting peak metamorphism occurred around c. 1.92 Ga. HREE-poor monazite with weighted mean ages of c. 1910–1895 Ma formed after the majority of garnet growth and either reflect the timing of melt crystallisation or recrystallisation during retrogression and fluid flow along major shear zones in the region. The P–T conditions and c. 1.93–1.90 Ga ages from the southeastern Taltson Domain are very similar to other rocks in the Dodge–Snowbird Domain of the Rae Craton, and demonstrate that the footprint of mid-P granulite facies metamorphism extends south and west into the southeastern Taltson Domain.

Multi-scale analysis and modelling of aeromagnetic data over the Bétaré-Oya area in eastern Cameroon, for structural evidence investigations

Abstract. This study was carried out in the Lom series in Cameroon, at the border with Central African Republic, located between the latitudes 5∘30′–6∘ N and the longitudes 13∘30′–14∘45′ E. A multi-scale analysis of aeromagnetic data combining tilt derivative, Euler deconvolution, upward continuation, and 2.75D modelling was used. The following conclusions were drawn. (1) Several major families of faults were mapped. Their orientations are ENE–WSW, E–W, NW–SE, and N–S with a NE–SW prevalence. The latter are predominantly sub-vertical with NW and SW dips and appear to be prospective for future mining investigations. (2) The evidence of compression, folding, and shearing axis was concluded from superposition of null contours of the tilt derivative and Euler deconvolution. The principal evidence of the local tectonics was due to several deformation episodes (D1, D2, and D4) associated with NE–SW, E–W, and NW–SE events, respectively. (3) Depths of interpreted faults range from 1000 to 3400 m. (4) Several linear structures correlating with known mylonitic veins were identified. These are associated with the Lom faults and represent the contacts between the Lom series and the granito-gneissic rocks; we concluded the intense folding was caused by senestral and dextral NE–SW and NW–SE stumps. (5) We propose a structural model of the top of the crust (schists, gneisses, granites) that delineates principal intrusions (porphyroid granite, garnet gneiss, syenites, micaschists, graphite, and garnet gneiss) responsible for the observed anomalies. The 2.75D modelling revealed many faults with a depth greater than 1200 m and confirmed the observations from reduced-to-Equator total magnetic intensity (RTE-TMI), tilt derivative, and Euler deconvolution. (6) We developed a lithologic profile of the Bétaré-Oya basin.

Lithostratigraphy of the Mesoproterozoic Donkieboud Granodiorite (Komsberg Suite), South Africa and Namibia

Abstract The Donkieboud Granodiorite pluton forms an extensive intrusion across the border region between South Africa and southeast Namibia. The mesocratic grey, weakly to moderately K-feldspar porphyritic biotite ± hornblende ± orthopyroxene granodiorite represents the most extensive member of the late- to post-tectonic Komsberg Suite (~1 125 to 1 105 Ma) which intruded as sheet-like bodies into the older high grade paragneisses and orthogneisses (~1 230 to 1 140 Ma) of the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Province. The Donkieboud Granodiorite comprises three main textural variations namely:a porphyritic to weakly porphyritic, relatively undeformed rock with randomly orientated ovoid and twinned feldspar phenocrysts;a weakly- to well-foliated gneiss with between 3 to 10% feldspar phenocrysts set in a medium-grained matrix anda patchy metamorphic charnockite variety. Large inclusions of the strongly foliated Twakputs (~1 210 Ma) and the Witwater (~1 140 Ma) garnetiferous granite gneisses occur within the Donkieboud Granodiorite and mafic xenoliths are common. The Donkieboud Granodiorite is variably deformed ranging from unfoliated to being gneissic. The foliation developed during its intrusion into an existing but waning regional stress field with the strain increasing towards the contacts with the surrounding country rocks. Subsequent km-scale open folding resulted in the reorientation of the gneissic foliation and locally, intense reworking of the fabrics along the margins of the folds. In places, the Donkieboud unit is crosscut by discrete mylonitic shears with a west to northwest trend. U-Pb zircon dating of the Donkieboud Granodiorite samples yielded concordia ages of between 1 118 and 1 107 Ma. Overall the Donkieboud Granodiorite has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic xenoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant amount of chemical weathering. The εNd values of -1.15 and -0.11 and TDM values of 1 615 and 1 505 Ma are typical of the Komsberg Suite and indicate a significant contribution of older crustal material to the magma of the Donkieboud pluton.

Soil water drives distribution of northern boreal conifers Picea abies and Pinus sylvestris

Considering the climate change the role of edaphic drivers for the distribution of cold-hardy conifers, Norway spruce (Picea abies (L.) Karst.), and Scots pine (Pinus sylvestris L.) is crucial. Even though both species reach latitude 70oN, P. abies forest line is located at exceptionally lower latitudes in Fennoscandia compared to P. sylvestris tree line. Hence the divergence in mafic (basic) and felsic (acidic) lithologies and thereby physicochemical properties of parent glacial tills may play a critical role in tree species distributions. The objective of this study was to explore soil water content (SWC) and nutrient regimes in Finnish Lapland by transecting the main lithological units; mafic Central Lapland Greenstone Belt (CLGB) in the south and felsic Lapland Granulite Belt (LGB) in the north. Eight one ha in size plots were sampled at 10 m intervals for soil chemistry and texture and measured for SWC, gamma-ray, electrical conductivity, temperature and matric potential. In binary decision tree (DT) analysis, soil physics explained the growth sites of P. abies and P. sylvestris with 97% accuracy, the most important variable being SWC. The constraint for P. sylvestris was SWC > 0.24 m3m−3. Based to the soil chemistry DT, 95.4% of the P. abies and P. sylvestris growth sites were correctly classified. The most important variables were Cr, Ti, Mn and P. Distribution of conifers follows lithology, such that P. abies occupies mesic-wet tills derived from ultramafic and mafic rocks of the CLGB, whereas P. sylvestris thrives only on dry and coarse-textured, often granitoid tills. We argue that dry and nutrient-poor tills of felsic garnet gneisses of the LGB constitute a dispersal barrier for P. abies. The above tree species specific constraints provide guidance for future dispersal models.

Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite–garnet gneisses

Because of the strong preference of fluorine (F) for “hydrous” magnesian silicate minerals, the temperature T(0F) given by the reaction (Bi–Gt) of Fe–Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the reciprocal effects between octahedral and hydroxyl site occupancies in biotite are ignored. In the granulites of southeastern Madagascar, widespread Mg metasomatism was associated with F-rich fluids. It is mainly manifested by the regional occurrence of diopside skarns hosting F-phlogopite and thorianite mineralisation. At first, biotite- and garnet-bearing granites and leucogneisses appear unaffected by this event. However, their biotites are rich in F [up to 6 wt%; XF = XF/(F + OH) = 0.69) and in Mg (XMg = Mg/(Mg + Fe) up to 0.8], which shows the tendency of equilibrium with the regional fluids. This equilibration leads to low values of Fe/Mg, which in turn cause strong underestimation of the temperature: T(0F) as low as 500 °C, whereas the temperature T(Ti) given by the TiO2 content of biotite is approximately 750 °C. The slightly modified thermodynamic model of Zhu and Sverjensky (Geochim Cosmochim Acta 56:3435–3467, 1992) for reciprocal solid solution is applied to a set of 19 samples to correct this underestimation and to quantify the effect of F on thermometry. The reciprocal activity coefficients for phlogopite (λ(Phl)) and annite (λ(Ann)) calculated by the model are added to the winTWQ software to obtain corrected values T(F) of the temperature. Similar values of ΔT = T(F) − T(0F) are obtained by (1) the linear fit of the whole set of the samples and (2) RTlnKλ = RTlnλ(Ann) − RTlnλ(Phl) = 33.417XF − 0.1092 (r2 = 0.9994) equation applied to a median sample: ΔT = 30–40, 60–70, 95–100, 160, and 200 °C for XF = 0.1, 0.2, 0.3, 0.5, and 0.7, respectively. The same calculations based on the Thermocalc thermodynamic data set lead to values of ΔT 1.5 times higher than those given by winTWQ. Thus, the results of these calculations confirm (for the first time from a set of natural samples) the validity of the thermodynamic model of reciprocal solid solution and the necessity of accounting for F in thermometry based on Fe–Mg exchanges involving biotite.

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

AbstractThe Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

Geochronological characterization of a transition zone between the Mozambique Belt and Unango-Marrupa Complex in SE Tanzania

We present geochronological and petrological data of orthogneisses, granitoids, and supracrustal rocks from SE Tanzania to characterize the transition zone between the Mesoproterozoic Unango-Marrupa Complex and the Neoproterozoic Mozambique Belt.Geochronological data (U-Pb zircon; LA-ICP-MS) indicate that the emplacement of orthogneisses (protolith) and granitoids in SE Tanzania occurred in Stenian, Cryogenian, and Ediacaran Periods. The magmatic emplacement ages are determined between 1056 ± 19 Ma and 554 ± 3.9 Ma. No older ages that that were determined.The Stenian orthogneisses (biotite granitic gneiss dates at 1056 ± 19 Ma and the associated migmatites, charnockites, amphibolite) represents Mesoproterozoic magmatic inliers in the Neoproterozoic rocks and the extension of the Unango-Marrupa Complex in SE Tanzania.Supracrustal rocks (marble and calc-silicate, garnet-biotite (hornblende) gneisses, quartzo-feldspathic gneisses, graphite schists, and quartzites) are interlayered with granitoids dated between 790 ± 8.7 Ma and 614 ± 8.1 Ma. The supracrustal rocks represent the upper part of the Mozambique Belt (Eastern Granulites/Cabo-Delgado nappes), and are possibly relics of continental shelf sedimentary deposits of the Mozambique Ocean.Upon assessing metamorphic ages (U-Pb zircon) in SE Tanzania we determined the Ediacaran metamorphic event dated between 606 ± 6.6 and 560 ± 37 Ma, which reworked the Stenian – Cryogenian crusts. The U-Pb zircon metamorphic ages are supported by a precise U-Th-Pb monazite metamorphic age at 589 ± 5 Ma obtained from a biotite garnet gneiss rock sample (amphibolite facies). The Ediacaran metamorphism in SE Tanzania was followed by the emplacement of biotite alkali granite at 554 ± 3.9 Ma, which probably marks crustal relaxation and extension after metamorphism as the result of collision or accretion.Magmatic rocks of the Mozambique Belt in SE Tanzania were emplaced during the East African Orogeny. Metamorphic reworking occurred during the final amalgamation of Gondwana between 600 and 500 Ma (Kuunga Orogenic Cycle) in line with tectono-magmatic history of the neighbouring Unango-Marrupa Complex, the Ubendian Belt, and the Irumide-Southern Irumide Belt.

Geology and geochemical constrains on the origin and depositional setting of the Kpwa–Atog Boga banded iron formations (BIFs), northwestern Congo craton, southern Cameroon

The Kpwa–Atog Boga (KAB) area lies within the Paleoproterozoic Nyong unit which defines the northwestern edge of the Archean Congo craton in southern Cameroon. Rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations (BIFs). The BIFs and its host rocks were subjected to amphibolite facies metamorphism. The Rare Earth Elements (REE) patterns and the discrimination ratios (e.g., Al2O3/TiO2 Cr/Th Th/Sc La/Sc) of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering. The KAB BIFs exhibit significant concentration of TFe2O3 + SiO2 (>93 wt%), and lower Al2O3 and TiO2 contents and high field strength elements (e.g., Hf, Nb, Ta, and Zr). This involves little contamination with continental detritus at their depositional environment. The Post Archean Australian Shale (PAAS)-normalized REE patterns of the KAB BIFs show enrichment in heavy REE and the depletion in light REE with positive La anomalies, which imply that these BIFs precipitated in ambient seawater. Moreover, the contribution of low-temperature hydrothermal fluids during the deposition of the studied BIFs were highlighted through their high superchondritic Y/Ho ratio (mean: 31.48), weak positive Eu/Eu∗(SN) ratio (mean: 1.83) and the low contents of transition metals. This suggests that the geochemical characteristics of the KAB BIFs and those of the Paleoproterozoic superior-type BIFs are almost similar. In summary, low-temperature hydrothermal fluids mixed with seawater contributed to the deposition of the KAB BIFs on the continental margin. This depositional setting similar to its host rocks is consistent with the Paleoproterozoic geological history recorded within the Nyong unit. This preliminary study reveals that the KAB BIFs are associated with low grade gold (<5 ppb), which could constitute further prospective mineralization besides the iron ore in the KAB area.

Petrologic and zircon U–Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta–igneous rocks, occurring as lenses within the meta–sedimentary rocks of the Badu Complex. However, metamorphic state of these meta–sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P–T loop. Results from integrated U–Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100–50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to granulite facies metamorphism of these supracrustal rocks occurred in the Early–Middle Triassic. Based on the presence of HP granulite facies pelitic rocks, it is inferred that significant underthrusting was involved during the Indosinian Orogeny which introduced these supracrustal rocks to lower crustal levels.

Zircon U–Pb dating and phase equilibria modelling of gneisses from Dinggye area, Ama Drime Massif, central Himalaya

The tectonic affinity of the Ama Drime Massif in central Himalaya remains a subject of debate. We provide evidences of the metamorphic conditions and new geochronological data for orthogneiss and paragneiss in the Riwu area, eastern side of Ama Drime Massif. The peak P–T conditions for garnet gneiss is 13–14 kbar and 750–765 °C using phase equilibria modelling. The zircon U–Pb dating results reveal that the orthogneiss has a crystallization age of 1,848 ± 12 Ma and zircon rims age of 1,767 ± 18 Ma. Three major age peaks characterize all detrital zircons from the paragneisses: 975, 1,161, and 1,749 Ma. These new data integrated with previous literature data indicate that (a) the Ama Drime Massif might be ascribed to the Greater Himalayan Series; and (b) the Himalayan terrane may have been located closer to both the India and Lhasa terranes during the amalgamation of the supercontinent Columbia.

Metamorphic and magmatic evolution of the Paleoproterozoic gneisses in the Sancheong area, Yeongnam Massif, South Korea, and their implications to the tectonics in the Northeast Asia

The Sancheong area in the southwestern Yeongnam Massif mainly consists of Precambrian paragneisses (quartz-feldspathic and garnet gneiss), orthogneiss (porphyritic gneiss) and anorthosite and is important for an understanding the tectonic evolution of the southern part of the Korean Peninsula and Northeast Asia. The Precambrian paragneisses in the Sancheong area underwent intermediate-P/T metamorphism (quartz-feldspathic gneiss; 730°C and 12.2kbar). The intermediate-P/T metamorphism was overprinted by regional low-P/T metamorphism (750–820°C and 7.0–9.0kbar) which was followed by amphibolite-facies retrograde metamorphism. In contrast, the orthogneiss only experienced the low-P/T (750–800°C and 8.0–8.7kbar) metamorphism followed by the amphibolite-facies retrograde metamorphism. Zircon U–Pb dating indicates that the intermediate-P/T metamorphism likely occurred at ca. 1880–1920Ma and that the low-P/T metamorphism occurred at ca. 1855–1859Ma. The whole-rock geochemical data and zircon U–Pb ages indicate that the protolith of the porphyritic gneiss and the anorthosite were emplaced in a post-collisional tectonic setting during ca. 1872–1880Ma and ca. 1860Ma, respectively. These data suggest that the Sancheong area underwent continental collision associated with the intermediate-P/T metamorphism (ca. 1920–1880Ma) and post-collisional igneous activity associated with the low-P/T metamorphism during the Paleoproterozoic (ca. 1856–1880Ma). Similar Paleoproterozoic tectonic events were recently reported from the eastern Cathaysia Block, suggesting that the Yeongnam Massif may have a close affinity to the eastern Cathaysia Block.

Evaluation of geomechanical and geochemical properties in weathered metamorphic rocks in tropical environment: a case study from Samanalawewa hydropower project, Sri Lanka

The effect of weathering on changes of physical and mechanical properties of rocks is a prime concern in the perspectives of geology and engineering. These properties have been studied mostly on weathered igneous and sedimentary rocks under humid climates. Studies on weathering of metamorphic rocks, especially under a tropical climate, are rare. This study evaluates change of physical, mechanical, chemical, and mineralogical properties of metamorphic rocks that weather under tropical climatic conditions. Samanalawewa hydropower project area was selected for this study, because rapid weathering of a metamorphic rock (sillimanite garnet gneiss) was observed in the project site. Fresh rocks that are subjected to weathering have reached to completely weathered condition in a time span of less than 25 years in this area. Visually assessed weathering grades were physically and mechanically evaluated using bulk density, equotip hardness, porosity, specific gravity, point load strength, and slake durability tests. Mechanical properties, especially point load strength, change rapidly at the onset of weathering, while chemical properties show significant changes at later stages of weathering. Mineralogical changes such as appearance of secondary minerals are at the latter part of weathering. Physical properties gradually change during weathering. The observed changes in physical, mechanical and chemical properties indicate that their variations during weathering are independent of lithology and climatic conditions.

Metamorphic evolution and zircon ages of pelitic granulites in eastern Hebei, North China Craton: Insights into the regional Archean P–T–t history

Archean supracrustal granulites occur as localized slivers or rafts in granitoids, and this particular occurrence of supracrustal granulites has not been much concerned from the perspective of metamorphic evolution. The pelitic granulites in eastern Hebei occur as slivers in TTG gneisses and charnockite. Three pelitic granulite samples involving orthopyroxene–garnet gneiss (JD1334), biotite–garnet gneiss (JD1453) and sillimanite–garnet gneiss (JD15126) were collected from Taipingzhai and Laolijia areas. Based on textural observations and mineral compositions, four generations of assemblages involving final, peak, inclusion and overprinting stages are identified. The observed final assemblage yield P–T conditions of 8.0–9.5kbar/870–890°C. The observed peak assemblage characteristic of the absence of the matrix biotite record condition of T>950°C with fixed P=9kbar. An isobaric cooling path from peak to final assemblage is constrained by mineral isopleths. The inclusion assemblages characterized by plagioclase inclusions with high XAn contents are predicted to be stable at low pressure and/or high temperature field. They form an anticlockwise P–T path. The overprinting assemblage characteristic of coronary garnet (g2) coexisting with fine-grained mineral aggregates defines P–T condition of 10–12kbar/830–880°C. The deposition age of the sedimentary protolith of pelitic granulites was ∼2.5Ga, coeval with the crystallization age of TTG gneisses. Both of them were later subjected to granulite facies metamorphism at ∼2.48Ga. The Archean anticlockwise P–T paths suggest a tectonic scenario to explain the particular occurrence of the Archean supracrustal granulites. Firstly, supracrustal sediments experienced thermal event under low pressures attributed to the intrusion of high-temperature TTGs, resulting in dehydrations of hydrous phases. Then, the dried metasediments interlayered with iron-bearing formations potentially denser than the surrounding lithologies tend to drop into the TTG magma ocean, causing the pressure-increase in the metasediments. Thirdly, the dropped metasediments experienced cooling process under lower crust in coupled with the crystallization of the surrounding TTG magma to form the observed final assemblages. Finally, they uplifted as a consequence of the doming processes in Archean terranes.

The evolution of a Precambrian arc-related granulite facies gold deposit: Evidence from the Glenburgh deposit, Western Australia

Gold deposits are rare in upper-amphibolite to granulite facies environments. Known examples commonly attract debate about whether they formed under these conditions or instead represent metamorphosed, metasomatic, or superimposed (retrograde) mineralization. The Glenburgh gold deposit is located in the Paleoproterozoic upper-amphibolite to granulite facies Glenburgh Terrane in the southern Gascoyne Province of Western Australia. Gold at the Glenburgh deposit is free and disseminated within quartz–biotite–garnet gneiss, amphibolite, and (post-gold) quartz–chlorite veins. No clear association with a specific host lithology has been identified and mineralization does not have a visually distinct proximal alteration assemblage.The rocks hosting the deposit represent a distinct sedimentary package that was deposited, mineralized, buried, and metamorphosed, all during arc magmatism. Features within the internal structure of gold grains, such as high-purity gold veinlets, incoherent twinning, and low silver content, suggest the gold has been through post-depositional processes such as metamorphism and deformation. Abundant sulfide minerals are interpreted to have formed by sulfidation of the host rock contemporaneously with gold mineralization, and the presence of rounded sulfide inclusions within garnet porphyroblasts illustrates the presence of a sulfide phase prior to peak metamorphism. Geochronology of zircon and monazite constrains the timing of mineralization to be younger than c.2035Ma—the maximum depositional age of the metasedimentary host rocks—but older than c.1991Ma—the peak of M1 metamorphism during the Glenburgh Orogeny; these events were synchronous with arc magmatism. Rocks at the Glenburgh deposit were likely deposited in a fore-arc or accretionary wedge, a favourable setting for porphyry Cu–Mo–Au, epithermal Au, polymetallic (Sn, W) skarn, and orogenic Au mineralization. Phase equilibria modelling of a pelitic migmatite constrains peak P–T conditions to be 865–885°C, 6.8–7.6kbar, consistent with elevated thermal gradients within the arc, followed by conductive cooling of arc magmas. Partial melting during peak M1 metamorphism possible caused gold remobilization. The lack of an alteration assemblage further suggests that the alteration assemblage and mineralization were recrystallized during deformation and metamorphism. However, increases in Ca and K abundance and magnetic susceptibility decreases toward mineralization, suggesting that they may constitute ore vectors.

Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao, from the Jining Group of the Khondalite Belt, North China Craton

AbstractThe Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the Tmax stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and &gt;840 °C/&gt;7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐Tmax decompression from the kyanite‐stability fields to the Tmax stage of 950–1020 °C/8–9 kbar and a post‐Tmax cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐Tmax cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant 207Pb/206Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐Tmax cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐Tmax decompression P–T path.

Zircon U–Pb ages and Hf isotope of gneissic rocks from the Huai’an Complex: Implications for crustal accretion and tectonic evolution in the northern margin of the North China Craton

The Precambrian tectonic evolution of the North China Craton (NCC) has long been a matter of debate. The Huai’an Complex, situated at the conjunction of the Khondalite Belt and Trans-North China Orogen (TNCO), undoubtedly can provide some key constraints on the geological evolution of the Khondalite Belt and the TNCO. We report zircon U–Pb ages and Hf-isotope results for four gneissic rocks and one Mesozoic mafic dyke in the Huai’an Complex to constrain the Precambrian tectonothermal evolution and crustal accretion in the northern margin of the NCC. The zircons from three dioritic to granitic gneisses and inherited zircons from the mafic dyke all yield Neoarchean to early Paleoproterozoic protolith ages of 2445–2533Ma, and register three later tectonomagmatic events with ages of ∼2360, ∼2150 and ∼1850Ma. The ∼2360Ma magmatism is recorded by a dioritic gneiss (2358±34Ma) from the Dongyanghe area and a granodioritic gneiss (2370±24Ma) from the Hunyuanyao area. The ∼2150Ma magmatism is recognized in a dioritic gneiss (2172±7Ma), granodioritic gneiss (2159±63Ma) and the Mesozoic mafic dyke (2144±57Ma). The ∼1850Ma magmatism is represented by zircon populations with ages of 1877±48Ma in the dioritic gneiss, 1844±69Ma in the granodioritic gneiss and ∼1800Ma in the mafic dyke. These Neoarchean to Paleoproterozoic ages reveal that the Huai’an Complex underwent a successive, complicated tectonomagmatic evolution, similar to the Hengshan–Wutai–Fuping Complexes, which represent a long-lived Neoarchean to Paleoproterozoic magmatic arc. In contrast, a sillimanite–garnet gneiss from the Kouzicun area yields completely different zircon U–Pb ages with peaks at 2056±32Ma, 1952±25Ma and 1901±25Ma. These three age groups suggest that the Kouzicun area experienced a tectonothermal evolution diagnostic of the Khondalite Belt, including protolith ages (2100–2000Ma), peak metamorphism (∼1950Ma) and post-orogenic exhumation (∼1900Ma). Thus, the geological boundary between the Khondalite Belt and the TNCO can be constrained to a position between the Kouzicun and Hunyuanyao areas. Coupled with zircon U–Pb ages, zircon Hf isotopic results reveal that ∼2500Ma mantle-derived mafic magmas not only provided enormous heat for partial melting of the 2.7–2.8Ga juvenile crust to form widespread TTG rocks, but also contributed directly to the generation of dioritic rocks. The Huai’an Complex in the northern margin of the NCC witnessed two Neoarchean episodes of continental-crust growth at 2.7–2.8Ga and at 2.5Ga.

Late Neoarchean crustal evolution of the eastern North China Craton: A study on the provenance and metamorphism of paragneiss from the Western Shandong Province

Metamorphosed clastic rocks provide important constrains on the composition of upper continental crust. Here we report and characterize a sequence of Neoarchean paragneiss in association with migmatite from the Qixingtai area in Western Shandong Province (WSP) in the North China Craton (NCC), that represent metamorphosed moderate- to fine-grained clayey clastic rocks.The garnet from the paragneiss is characterized by nil or weakly compositional zoning. The biotite shows low TiO2 content (<2wt.%) and high Fe/[Fe+Mg] molar ratios (0.45–0.48). The P–T estimates suggest that the paragneiss sequence was metamorphosed at 5.5–5.7kbar and 678–679°C. The detrital zircons from the sillimanite garnet gneiss yield concordant 207Pb/206Pb ages from 2.78 to 2.53Ga (discordance ≤15%), suggesting that the protolith sediments were deposited after 2.53Ga. The overgrowth rims on the detrital zircons of the paragneiss and those from the leucosome of the migmatite show similar low Th/U values, moderate Th and high U and common lead contents, indicating that the metamorphic and anatectic zircons formed under fluid-rich conditions. The intrusive contact between the massive monzogranite and the banded migmatite suggests that the regional metamorphic/anatectic event occurred before the emplacement of the monzogranite. However, the upper intercept ages, defined by the overgrowth rims of the pre-existing zircons from the leucosome, are broadly similar to those of the crystallization of the magmatic zircons from the monzogranite within errors at ∼2.50Ga.The paragneiss exhibits SiO2 contents in the range of ∼58wt.% (garnet gneiss)–∼70wt.% (sillimanite gneiss), and negative correlations with Al2O3 (23.4–15.4wt.%), TiO2 (0.90–0.35wt.%), MgO (4.5–1.1wt.%), FeOt (8.3–2.6wt.%) and Cr (382–131ppm). The REE patterns also show a systematic change from garnet gneiss to sillimanite gneiss. In trace element discrimination diagrams, the paragneiss plots between the end-members of komatiite/basalt and TTG/granite, suggesting that the sediments sourced from a mixture of ultramafic–mafic and felsic protoliths. Combined with the geochronological data, we suggest that the 2.75–2.71Ga komatiite–tholeiite sequence of the Taishan association and TTGs in the central terrane and the 2.56–2.53Ga TTGs in the southwestern part of the WSP as the potential provenance for the clastics.The protolith of the paragneiss in the Qixingtai area belong to the sedimentary sequence of the Taishan association. Spatially associated coeval arc-like assemblage and other geological evidence suggest that, (1) the metasediments were likely deposited in a back arc basin, (2) the upper amphibolites-facies metamorphism, constrained by the mineral assemblages in the paragneiss, might be related to the closure of the basin, and (3) the generation of large volumes of matured K-rick continental crust occurred not earlier than 2.53Ga in the WSP granite–greenstone belt.